Nitrogen-purged storage desiccators provide a clean, dry storage environment for stored, moisture-sensitive items. Configured for use with a nitrogen purge gas flow controller, such as one sold by Terra Universal, Inc. of Fullerton, Calif., under the name ISODRY™, desiccators provide a continuous purge of clean, dry nitrogen to flush out moisture-laden air.
For decades, desiccator storage has been a common practice in many industries, including semiconductor, electronics, aerospace and medical device manufacturing. As critical components become smaller and more sophisticated, their susceptibility to moisture damage increases. In recent years, desiccators have been widely used in bio-pharmaceutical manufacturing to inhibit moisture-related degradation of drugs and biological samples. Once absorbed by sensitive components, water creates a number of potentially disastrous conditions with costly effects. Even minute traces of oxidation, the most notorious result of moisture exposure, can degrade soldering and other manufacturing processes. Because water dissolves ionic contaminants, it also alters the conductivity of the material, which in turn can degrade electrical function. Water also combines with other materials, causing harmful chemical reactions that degrade pharmaceutical samples and chemical mixtures.
One common method of dealing with moisture contamination is to remove it prior to each manufacturing step. Although vacuum processing and bake-and-bag methods of sample drying accomplish this end, these operations slow down production, particularly if they must be repeated several times in the course of circuit manufacturing. Further, these baking and sealing processes themselves expose parts to thermal extremes that can cause damage.
Desiccant dryers avoid some of these drawbacks, but introduce others. Such desiccant dryers remove moisture from air (or other process gas) inside the desiccator chamber and often feature dual module designs that perform online drying and offline desiccant regeneration simultaneously for continuous operation. Such dryers can be effective, but they require heating/drying components that may not be reliable or that may affect stored components. It can take many hours to reduce ambient conditions to a relative humidity of ten percent water vapor at room temperature. Their complexity and high operating costs makes them prohibitively expensive for long-term storage applications.
As an alternative to desiccant dryers, nitrogen-purged desiccator systems maintain dry conditions relatively cheaply and conveniently. The fundamental principle of nitrogen-purged desiccator cabinets is to displace moist air with nitrogen gas. Such systems employ one or more chambers in which the moisture-sensitive content is stored. A continuous purge of nitrogen gas continuously enters the cabinet, displacing any water vapor, and exits through an exhaust valve preferably on a side opposite a gas inlet. Displacement (sweeping), which depends on laminar flow, is the common method employed, although some mixing may be achieved. Consequently, the current technologies discussed above fail to achieve laminar flow effectively. The concept, design, and construction of conventional nitrogen purge desiccators thus incorporate conflicting technologies of displacement and mixing, resulting in inefficiencies.
Figure A depicts a prior art desiccator a having a single displacement channel DC, where dry nitrogen gas N2 is released into the highest chamber 1, to be distributed in a downward flow of gas through the stacked-up vertical chambers 1-5. This is the most common and believed to be the most economical configuration for desiccators. The chambers 1-5 are connected in series and in communication with one another through perforations P in the floors FL of the chambers 1-4, so gas flows in series from a higher chamber into a lower chamber. The nitrogen gas N2 entering the highest chamber 1 moves downward mixing with the moist air in the chambers 1-4. The most basic configuration utilizes a manually-adjusted gas inlet connected to a flow meter FM. Upgraded models of the flow meter FM use an automatically controlled humidity module in which the user can specify a predetermined humidity set point as a percent of water vapor (RH %). When a humidity sensor (not shown) detects a moisture reading above the set point, a solenoid (not shown) opens a valve to release purge gas until the relative humidity set point is reached.
As shown in Figure A, the dry nitrogen gas N2 is flushed into the series of chambers 1-5 from a single point inlet I to displace humid air through a single point outlet O. The chambers 1-5 are in communication with each other so gas flows in series from one chamber to the next. The graph of FIG. 26 shows the performance and concentrations of moisture within each chamber of Figure A. As shown in the graph of FIG. 26, humidity concentrations are not uniform. Furthermore, the humidity sensor (not shown) for the nitrogen purge controller may sometimes provide a misleading impression of the desiccator's overall humidity. Thus, the purge cycle may be discontinued prematurely. FIG. 27 shows the performance and concentrations of moisture within each chamber 1-5 shown in Figure A. The fluctuation shown in the graph of FIG. 27 seen in chamber 5 is a result of a controller unit (not shown) shutting on and off.
Figure B depicts a prior art, multi-chambered desiccator b where the humidity of each chamber 1-5 is purged with dry nitrogen gas N2 by sweeping the purge gas through individual chambers 1-5 in an attempt to achieve a laminar flow. In this embodiment humidity=concentrations are not uniform within the individual chambers. In order to maximize displacement efficiency, a perforated plenum chamber PC1 is utilized to provide a continuous, uniform gas flow to individual chambers. The chambers are connected in parallel with gas flow. They are not in communication with one another through their solid floors FL, and because of the positive pressure of the gas in the plenum chamber PC1, the gas does not flow between the chambers. Current state of the art designs utilize a door sensor DS (FIG. 6) on each door D of the chambers 1-5. The door sensor DS actuates a high-pressure purge whenever a door D is opened. The positive pressure within the chambers 1-5 inhibits moisture or contaminants from entering a chamber as gas flows out an open door D. For critical environment applications, multi-channel purge controllers are available, where each chamber has its own sensor and purge controller.